The disclosed invention relates generally to automatic test equipment (ATE) used for automated production line testing of multi-pin integrated circuits, and particularly to active load circuits employed in such test equipment. An active load circuit is connected to a pin that is being monitored for response to test signals applied to other pins of the integrated circuit device under test (DUT).
Automatic test equipment for automated testing of integrated circuits typically employ a large number of xe2x80x9cpin cardsxe2x80x9d that include circuitry for connection to a corresponding pin of an integrated circuit device under test. Each pin card includes a pin driver circuit that supplies a test signal to the associated pin, and an active load circuit that sources or sinks a current depending upon the output voltage of the pin. The pin driver and the active circuit are switchably connectable to a corresponding pin of the device under test, such that the pin is connected to either the pin driver or the active load at any given time. For reference, an active load can be considered as being in the ACTIVE mode when it is connected to the pin, and in the INHIBIT mode when it is disconnected from the pin.
The circuitry employed to switch a pin between the active load and the pin driver commonly includes Schottky diodes arranged in a bridge. One or more of the Schottky diodes is subjected to relatively large levels of reverse bias as the output voltage on a pin swings over a typical range such as xe2x88x922 to 7 volts. The large reverse bias can cause leakage currents as well as degradation of the forward action characteristics. To the extent that the leakage currents are directly connected to the pin, the leakage currents must be extremely low in the INHIBIT mode, but can be acceptable in the ACTIVE mode so long as the leakage does not affect the functionality of the active load circuit.
In high speed applications, the leakage and/or reliability of Schottky diodes that are on-chip with the active load circuit can be unacceptable. This has required the Schottky diodes to be a separate integrated circuit which is ineffecient and more costly.
An active load circuit is described that includes a current source; a current sink; a current switching switching circuit having current source and current sink nodes respectively connected to the current source and the current sink; and a control circuit for controlling the current switching circuit with a differential voltage that is limited in amplitude and of the same polarity as the difference between a fixed reference voltage and a pin output voltage of a device under test.